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5 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24). Differences 27, 33, 34). The creation of traffic and congestion during in work zone designs, quality of traffic control device main- work activities is usually hypothesized as the major reason for tenance, types of work performed, and other roadway and the observed increase in rear-end crashes. Traffic and con- traffic characteristics probably contribute to the varying re- gestion can be created by temporary lane closures, other re- sults observed. In addition, recent studies have shown that the ductions in roadway capacity (i.e., narrowed lanes, lane shifts, relationship between work zone crash likelihood and road- etc.), drivers who are confused about their proper travel path way (i.e., average daily traffic [ADT], lane and shoulder and slow down, or the movement of construction equipment widths, etc.) and work zone characteristics (i.e., duration, into and out of the work area. length, etc.) are nonlinear (24, 25). As to whether such crash increases are more significant at Nighttime versus Daytime night, the evidence is less clear. Some studies have found that Work Zone Crashes nighttime crashes in work zones increase by a greater per- centage than daytime crashes (21, 26, 27), but other studies Decisions about whether to perform work in travel lanes have found the increases in daytime and nighttime crashes to during daylight hours or at night should be based, in part, on be similar (8, 19, 28). which approach is likely to yield the lowest crash costs over In essentially all studies described above, the changes in the duration of the project (35). Previously, only three small crashes were computed over the entire duration of a long- studies were identified that attempted to examine this question term roadway rehabilitation or reconstruction project, dur- directly. In the first study, researchers in California examined ing times when work was occurring as well as when the work eight construction projects where work was performed at area was inactive. Projects involving major rehabilitation or night to minimize traffic disruptions (36). Researchers found reconstruction of the roadway often require temporary that crash rates per million-vehicle-miles (mvm) of travel ex- degradations in roadway geometry such as narrowed lanes, posure were consistently and substantially higher at night shortening of entrance or exit ramps, and the placement of when work activity was present. The magnitude of increases temporary concrete barriers immediately adjacent to the ranged from 67 to 156 percent, with an overall average crash travel lanes. These degraded geometric changes are left in rate increase of 87 percent. Researchers further stratified the place when work is occurring as well as when the work zone data based on whether or not travel lanes were closed during is inactive. Consequently, the changes in crashes reported in the period of work activity each night. They found the crash these studies actually represent the combined effect of both rates at night during lane closures to be an additional 75 per- the degraded geometric conditions in the work zone and the cent higher than during periods of work activity at night influence of work activity itself. when travel lanes were not required to be closed. Researchers Although some consensus exists that overall crash rates also examined crash rates on the basis of crashes per million increase during roadway repair and reconstruction activities hours (per mile per lane to normalize data between projects) (even if the amount of the increase is of some debate), the and found the rates to increase an average of 122 percent dur- literature is less definitive with regards to whether or not ing periods of night work activity (as compared to non-work crashes tend to be more severe, less severe, or as severe as hours). Unfortunately, the analysis could not answer the ul- under non-work zone situations. Several researchers have timate question as to whether or not crash rates per mile of concluded that work zone crashes are no more severe than work zone and/or crash costs would have been higher or non-work zone crashes (8, 9, 13, 18, 19, 29, 30). However, lower than this had the work been performed during the day others (21, 31, 32) have found that work zone crashes were more instead. severe than non-work zone crashes or increased more signif- A second study used Illinois fatal work zone crashes and icantly than property-damage-only (PDO) crashes in the work national estimates of work activity occurring during daytime zone in the databases they examined. Once again, differences and nighttime periods to assess the relative safety of day- in the type of analysis used and the characteristics of roadways time and nighttime work zones (37). The estimates of exposure and work zones examined may be at least partially responsible came from a sampling process of scheduled work activity in- for the divergent findings. formation posted online by state and local transportation Despite the difficulties in properly attributing changes in agencies (38). Based on their analyses, the researchers con- crashes to the effects of the work zone design or to the actual cluded that night work was five times more hazardous than presence of work activity itself, rear-end crashes are often daytime work activity. However, the lack of actual exposure found to be overrepresented in the crashes that do occur. The data from Illinois work zones for use in the comparison was percentage of work zone crashes that involve rear-end impacts noted as a key limitation of the analysis. increased 7 to 83 percent, depending on the study cited and A third study examined five urban freeway reconstruction location of the work zone examined in that study (8, 18, 21, projects in Texas where all work activities that required lane